Optical fibers for use in optical fiber communication systems that are generally widely used have a structure in which the outer circumferential surface of one core is surrounded by a cladding. Optical signals propagate through the inside of this core, and thus information is transmitted.
Nowadays, with the wide spread use of optical fiber communication systems, information volumes to be transmitted are dramatically increased. In order to implement an increase in the transmission capacity of such optical fiber communication systems, a multicore fiber is known in which the outer circumferential surfaces of a plurality of cores are surrounded by a cladding. In accordance with the multicore fiber, signals can be transmitted using light beams propagating through the plurality of cores. Thus, the information volume that is transmittable through one optical fiber can be increased. Such a multicore fiber is disclosed in Patent Literature 1 below, for example.
As also described in Patent Literature 1 below, in the multicore fiber, a part of a light beam is sometimes overlapped with a part of another light beam, which propagate through cores adjacent to each other, sometimes resulting in inter-core crosstalk. An example of a method of reducing inter-core crosstalk that is considered is to increase the pitch between the cores adjacent to each other. The inter-core crosstalk is determined by the integration of the overlap of the light beams propagating through the cores adjacent to each other. Thus, the core pitch is increased to decrease the overlap, and this reduces inter-core crosstalk. The inter-core crosstalk can also be reduced by surrounding the core with a low refractive index layer having its refractive index lower than the refractive index of the core or the cladding, the low refractive index layer being formed of glass or a void. The core is surrounded by the low refractive index layer to decrease a spread of the light beams propagating through the cores in the radial direction. Thus, the overlap of the light beams propagating through the cores adjacent to each other is decreased, and this reduces inter-core crosstalk.    [Patent Literature 1] JP 2012-211964 A
However, when the core pitch is increased in the multicore fiber, the number of cores provided in one multicore fiber has to be decreased in the case in which the fiber diameter is constant, and the information volume that is transmittable is decreased. In the case in which the core is surrounded by a low refractive index layer (a trench), it is known that a decrease in the core pitch increases the cutoff wavelength of the inner core surrounded by a plurality of cores with a trench (the cores surrounded by a trench). When the cutoff wavelength is excessively increased, a needless higher-mode light beam is prone to propagate. When the needless higher-mode light beam propagates, the higher-mode light beam possibly causes crosstalk or multi-path interference (MPI). Therefore, an increase in the cutoff wavelength is reduced to a predetermined range. From the viewpoint of reducing an increase in the cutoff wavelength, the core pitch is increased. However, when the core pitch is increased, the information volume that is transmittable through one multicore fiber is decreased as described above.
As described above, in the previously existing multicore fiber, the information volume that is transmittable is sometimes decreased in order to reduce an increase in the cutoff wavelength and reduce inter-core crosstalk.
Optical fibers now commercially available as products have prescribed cutoff wavelengths according to the two-meter fiber cutoff wavelength and the 22-meter cable cutoff wavelength. That is, similarly to optical fibers now commercially available as products, the previously existing multicore fiber is designed to have core pitches and any other parameters in such a manner that cutoff wavelengths at which a light beam propagates two meters or 22 meters have predetermined values.
However, in the case of performing long-haul communications, such as metro networks and basic trunk networks, optical fibers in a length of at least about one kilometer are used. In the case of performing long-haul communications, optical fibers are laid in lengths ranging from one kilometer to a few thousand kilometers. However, it is difficult to implement this laying using one optical fiber. Thus, a plurality of optical fibers is connected in intervals of at least about one kilometer. Here, the present inventors found that in order to provide a multicore fiber suitable for long-haul communications, a suitable multicore fiber can be provided only under the condition in which the cutoff wavelength at a point in time when a light beam propagates one kilometer has a predetermined value. Therefore, one or more embodiments of the present invention provide a multicore fiber suitable for long-haul communications.